-LRB- CNN -RRB- -- The stubborn Rim Fire , one of the largest wildfire in California 's history , is 70 % contained , the U.S. Forest Service said Monday night .

With rain and cooler temperatures in the forecast , firefighters who have toiled tirelessly for days will likely make further gains in the coming days .

Still , officials do n't expect full containment until September 20 .

The Rim Fire started August 17 and swallowed more than 235,000 acres -- making it the fifth-largest wildfire in California history . It has cost the state more than $ 39 million to date .

Although the fire has consumed tens of thousands of acres inside Yosemite National Park , it has so far had little or no direct impact on Yosemite Valley , a popular spot for tourists and home to many of the park 's iconic attractions , including the El Capitan rock formation .

Authorities do n't know what started the Rim Fire -- although the fire chief in the town of Twain Harte said illegal marijuana growers could be the unintentional culprits .

`` It might be some sort of illicit grove , marijuana grow-type thing . '' Todd McNeal told a meeting on August 23 . The video of the meeting was posted on YouTube and picked up steam recently .

Calls to the Twain Harte fire department were not answered Monday night . But Mark Healey with the Rim Fire Information Line dismissed the suggestion , calling it `` rumors . ''

The fire , he said , was still under investigation .

What to know about wildfires

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Officials do n't expect full containment until September 20

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Twain Harte fire chief says marijuana growers could be to blame

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Rim Fire officials called the suggestion `` rumors ''